Solutions of lithium hexafluorophosphate (LiPF.sub.6), lithium hexafluoroarsenate (LiAsF.sub.6), lithium hexafluoroantimonate (LiSbF.sub.6) and lithium tetrafluoroborate (LiBF.sub.4) dissolved in a variety of organic solvents have found utility in primary and secondary lithium batteries. In particular, nonaqueous electrolytic solutions comprising lithium hexafluorophosphate exhibit high electrochemical stability and conductivity. Nevertheless, the lithium hexafluorophosphate used in electrolytic solutions is susceptible to both thermal decomposition and hydrolysis, each of which are catalyzed by the presence of acidic impurities in the lithium salt or solution.
The thermal decomposition of LIPF.sub.6 occurs at elevated temperatures according to Equation 1. ##EQU1## The thermal decomposition of LiPF.sub.6 is typically minimized by storing the lithium salt and the lithium salt solutions under refrigerated or sub-ambient conditions.
The hydrolysis of lithium hexafluorophosphate occurs according to Equations 2-5. ##EQU2## Hydrolysis generally occurs because of the presence of moisture and acidic impurities in the lithium salt or solution. Therefore, it is preferred that the lithium salt solution be free of all acidic impurities in order to achieve high levels of stability and performance. Nevertheless, hydrogen fluoride is a reactant in the formation of commercial grades of lithium hexafluorophosphate and trace amounts of at least 100 ppm generally remain in the lithium salt. See, e.g., Introduction to Hashimoto, distributed by Biesterfeld U.S., Inc. (1994).
As shown in Equations 2-5, once hydrolysis is initiated (Equation 2), the rate of hydrolysis progressively increases because hydrogen fluoride, which catalyzes the reaction, is formed as a by-product of the hydrolysis reaction. Furthermore, the Li-P-O intermediates (e.g. LiPO.sub.2 F.sub.2, LiHPO.sub.3 F, LiH.sub.2 PO.sub.4) are more easily hydrolyzed than LiPF.sub.6 and thus facilitate the accumulation of HF in the lithium salt solution. Therefore, the lithium salts and lithium salt solutions preferably should be free of water, hydrogen fluoride and the Li-P-O intermediates to increase the stability and the performance of the lithium salts and lithium salt solutions. Additionally, because hydrogen fluoride and other acids are also highly detrimental to the function of the active components of lithium and lithium ion batteries, these acidic species should be removed.
One method of removing acidic impurities and thus preventing the decomposition and hydrolyzing of lithium hexafluorophosphate is to treat the salt and/or solution with a base and then maintain the salt and/or solution under basic conditions. For example, U.S. Pat. No. 5,378,445 to Salmon et al. describes the use of a base such as ammonia to prevent the acid-catalyzed decomposition of lithium hexafluorophosphate. Nevertheless, the presence of ammonia in the electrolytic solution may be detrimental to battery performance. Furthermore, the reaction products of ammonia, e.g., NH.sub.4 F formed by the reaction of hydrogen fluoride and ammonia, may be detrimental to battery performance. Therefore, ammonia and its reaction products generally must be removed from the final product.